(a) Field of the Invention
The present invention relates to an electrode and a manufacturing method thereof for a secondary cell (rechargeable battery). More particularly, the present invention relates to an art that introduces a porous insulating layer including an inorganic compound, etc., to the surface of the electrode and simultaneously dries an electrode layer slurry and the porous insulating layer by applying the electrode layer slurry in order to prevent the binder of the porous insulating layer from blocking pores in a state in which the electrode layer slurry has not been dried after the electrode layer slurry is applied.
(b) Description of the Related Art
A secondary cell (rechargeable battery) is generally a chemical cell that can be repeatedly charged and discharged by reversibly interconverting chemical energy and electrical energy, and includes four fundamental elements such as a positive electrode, a negative electrode, a separator, and an electrolyte.
The positive electrode and the negative electrode actually convert and store oxidation/reduction energy, and have a positive potential and a negative potential, respectively. The separator is located between the positive electrode and the negative electrode in order to electrically insulate them and provide a movement path for a charge, and the electrolyte acts as a medium for charge transmission. An electrode material is the actual reacting material among the component materials is also called an active material.
A positive electrode material can intercalate, e.g., lithium ions, and a negative electrode material can be a compound in which, e.g., lithium metals, alloys, and lithium ions can be intercalated. The separator typically has a shape of a porous polymer layer or a non-woven fabric and is a single layer or is formed with multiple layers.
If a liquid electrolyte including an organic solvent and a salt is used, a movement path for ions moving between the positive electrode and the negative electrode during charge and discharge of the cell can be provided. The organic solvent improves the degree of dissociation for ions by increasing the polarity of the electrolyte. Further, it is preferable that a material with high polarity and that is nonreactive to lithium metal is used as the organic solvent in order to facilitate the conductivity of ions by lowering the local viscosity near the ions.
Hereinafter, the conventional electrode manufacturing process will be described.
The electrode manufacturing process generally includes five steps such as material weighing, mixing, coating, drying, and pressing. The material weighing is a step in which the electrode element materials according to a predetermined ratio are weighed, and the mixing is a step in which the electrode element materials are dissolved, dispersed, and kneaded in accordance with a predetermined order.
The materials are generally in a slurry form after the mixing process, and are then input to the coating process. That is, the mixed slurry includes the electrode element materials and a dispersion medium, and the electrode element materials include different electrode materials (active materials), a conductive material, a binder, and additives.
The coating and drying process, as shown in FIG. 1, is performed in such a manner that the electrode that is uncoiled from an unwinder 10 is coated while passing a coater 20 and is then wound again on a rewinder 40 after moving through a dryer 30.
The mixing process is a process in which the electrode element materials are dissolved, dispersed, and kneaded, and the levigated electrode element materials such as electrode materials, conductive materials, and polymer binders are also dry-mixed in a sealed container and then dissolved or dispersed in a solvent of the polymer binders or a dispersion medium.
The coating and drying process is a process in which the slurry formed by mixing the above materials is coated on a current collector and dried, and then an electrode only including solid content is obtained. The electrode is manufactured in a bar shape. The current collector is a material that collects electrons obtained from an oxidation/reduction reaction of the electrode material and causes them to flow into an outer conductor line. The pressing process is a process in which a predetermined pressure is exerted on the completely dried or semi-dried electrode obtained from the coating and drying process.
As described above, the electrode is finally finished through material weighing, mixing, coating, drying, and pressing steps.
However, there are stability issues with respect to manufacturing of secondary cells, and in particular, lithium secondary cells manufactured via processes having a high energy density and utilizing combustibles such as an organic solvent.
As a result of the instability, major accidents have occurred from an abnormally high temperature caused by a short circuit between the positive electrode and the negative electrode. That is, under normal conditions, the separator located between the positive electrode and the negative electrode maintains an electrical insulation state, but as there is a limit in the amount of energy a conventional separator can absorb, if the cell is overcharged or overdischarged, if dendritic growth of the electrode material or a foreign material causes an internal short circuit, if a sharp object such as a nail or a screw penetrates the cell, or if the cell is deformed by an external force, the temperature may be dramatically increased thus causing, e.g., an explosion.
A micro-porous layer consisting of a polyolefin resin is usually used as a separator, but the heat-resistance property of this layer is insufficient because the heat-resistance temperature is only about 120 to 160 degrees Celsius. Therefore, if an internal short circuit occurs, the separator contracts and the short circuit portion is enlarged, and then a problem that thermal runaway in which much more reaction heat is generated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.